## Description 

This is the second part of Garbage Collection which implements the
following ticked next steps as outlined on the previous PR

- [x] BlockManager to respect the gc_round when accepting blocks and
trigger clean ups for new gc rounds
- [x] Skip blocks that are received which are <= gc_round
- [x] Not propose ancestors that are <= gc_round
- [x] Subscriber to ask for blocks from `gc_round` when
`last_fetched_round < gc_round` for a peer to prevent us from fetching
unnecessary blocks

Next steps:

- [ ] Re-propose GC'ed transactions (probably not all of them)
- [ ] Implement new timestamp approach so ancestor verification is not
needed
- [ ] Harden testing for GC & edge cases

## Test plan 

CI/PT

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## Release notes

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